An experimental study of the interaction of water with granitic melt

[摘要] The nature of water in granitic melt is investigated through a variety of experimentaland analytical techniques. The knowledge of the presence of two dissolved melt species(hydroxyl and molecular water groups), and the precise determination of theirconcentrations with infrared spectroscopy, is used in conjunction with vacuum extractionand hydrogen isotopic techniques to determine, (1) the solubility of water in granitic meltsas a function of pressure at 850°C, (2) the speciation of water in granitic melt as a functionof temperature, and (3) the fractionation factors which govern the partitioning of hydrogenisotopes between water vapor and granitic melt.Natural obsidian starting materials were held at 850°C and pressures ranging from200 to 1600 bars in the presence of excess vapor. Samples were rapidly quenched toglasses and their water contents were determined using vacuum manometry and infraredspectroscopy. The results of these experiments demonstrate a progressive increase insolubility with pressure, but suggest that increasing dissolution of water has no influenceon the total volume of the melt (the partial molar volume of water in these low pressuremelts is near zero). The solubility measurements can be used to model shallow leveleruptive cycles which involve crystal fractionation, volatile saturation, onset of eruption,and influx of new magma to repeat the cycle. Preliminary experiments on the temperaturedependence of water solubility suggest a dependence as high as 1 wt % per 200°C at 1600bars. This result supports the suggestion that mafic intrusion into silicic magma chambersinduces supersaturation of the felsic liquid and subsequent explosive eruption.Hydrated obsidian chips were held at temperatures ranging from 450 to 600°C todetermine the equilibrium speciation of water in the melt. These data show distinct nonidealbehavior, In K_(eq) decreases with increasing water content (K_(eq) = the equilibriumconstant for the reaction involving dissolved water species). Relaxation analyses are usedto demonstrate that these rapidly-quenched glasses preserve their high-temperaturespeciation. This analysis has been used to demonstrate that melts held at 850°C do notpreserve their equilibrium speciation on quench. Intersection of the equilibrium isothermswith the fictive temperature curve recorded by the 850°C samples provides identification ofthe last-equilibrated temperature for the quenched glasses. This information, coupled withthe speciation recorded in glasses quenched at a slower rate, is used to extract the viscosityof hydrous rhyolitic liquids as a function of temperature. This formulation is compared toempirical methods for determining melt viscosities. The equilibrium speciation data is usedto formulate a regular solution model to determine the speciation of water in rhyolitic meltsat any temperature and water content.The hydrogen isotopic composition of water evolved from quenched granitic meltsis compared to the isotopic composition of their equilibrium vapors. The bulk partitioningof hydrogen isotopes between melt and vapor varies as a regular function of the totaldissolved water content, suggesting that two independent fractionation factors control theirequilibrium. The fractionation factor between vapor and hydroxyl groups (≈1.040 ± 0.05‰) is significantly greater than the fractionation between vapor and the molecular waterspecies (≈1.00 ± 0.10 ‰). These fractionation factors can be used to explain thedegassing trends measured in natural igneous environments. The magma chamber beneathMono Craters, CA may have experienced a significant amount of closed system degassingbefore undergoing open system degassing late in its history.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Geology